From the results of the experiments carried out for clarifying the causes of picture image blurring, the present inventors made the following discoveries:
(1) Although the phenomenon becomes more remarkable as the copying operation is repeated as described above, this direct cause is to subject the surface of the photoreceptor to degeneration by corona discharge. This fact has been confirmed from the following experiments.
A commerically available copying machine was modified so as to be subjected to only (a) the cycle of initial corona charge and alternating current corona discharge; (b) the cycle of light irradiation; (c) the cycle of magnetic blush development; or (d) the cycle of cleaning, and was repeatedly subjected to each cycle under normal conditions until 10000 copies were obtained. Thereafter, the copying machine was operated under the conditions of 30.degree. C. and 85% relative humidity (RH). The resultant copies were evaluated with respect to picture image blurring. As the result, it was found that the picture image blurring occurred only when a-Si photoreceptor was repeatedly subjected to corona discharge.
For comparison, a commercially available Se photoreceptor was repeatedly subjected to corona discharge. Under the same conditions as above, picture image blurring occurred.
From these facts, it is supposed that in the case of a conventional Se photoreceptor, its surface layer is gradually scraped so as to always keep a fresh surface via the serial copying cycles, particularly development and cleaning cycles due to the relatively low surface hardness, even if picture image blurring occurred by corona discharging and that in the case of an a-Si photoreceptor, its surface layer, degenerated by corona discharging, cannot be removed via the developing and/or cleaning cycles due to its very high surface hardness and therefore the degenerated layer accumulates as copying operations are repeated.
(2) Using the a-Si photoreceptor on which picture image blurring occurred under the conditions of 30.degree. C. and 85% RH, after copying operations were repeated under normal temperature and humidity conditions, until 10000 copies were obtained, the following experiments were carried out.
By washing the photoreceptor with trichloroethylene or forcedly filming with fresh toner on the surface of the photoreceptor, the occurrence of picture image blurring could be prevented.
From the result, it is found that picture image blurring is caused by the degeneration of the filmy toner and that picture image blurring cannot occur either by removing the degenerated toner or by filming with a fresh toner.
From the above findings, it is believed that the filmy toner serves as a layer for protecting the a-Si photoreceptor from corona charging and that the filmy toner is necessarily refreshed for preventing picture image blurring. Therefore, the preferable toner should have a high corona resistance and be easily made to be filmy.